Drilling equipment of this type known in the art (German open application No. 31 41 856) has the particular advantage that no rigid drilling rod is required. The flexible high-pressure hose pipe allows boring in a single operation from narrow mine openings of drill holes in any direction and of any length limited only by the length of the high-pressure hose.
But the known drilling equipment of this kind has the disadvantage that drill holes made by it must have a very large diameter and can be kept directionally constant only with difficulty. The relatively large diameter is necessary because in the case of the known drilling equipment the turning mechanism and the advance mechanism are located in the deepest point of the drill hole and require the corresponding space. Because of the relatively large diameter the known drilling equipment can be used economically only in easy drillable rocks, for instance for drilling in coal-bearing strata, for making water-injection holes, but cannot be used in hard rock for making small-diameter rock-anchoring holes or blast holes. The directional stability is deficient, because the high-pressure hose pipe cannot provide a guide for the annular bit.
According to German open application No. 30 29 963 drilling equipment can have a small-diameter annular bit for cutting the rock with high-pressure water-jet nozzles. In this case, however, rotationally driven hollow and rigid drill bars are used for feeding the high-pressure water and therefore, this equipment has no significant advantage over the commonly known drilling equipment with rigid drill bars.
Finally, according to U.S. Pat. No. 4,057,115 and German open application No. 30 43 512 drilling equipment can use a flexible shaft instead of drill bars, said shaft stiffening to a rigid bar under torque. For this purpose, the flexible shaft consists of an inner flexible element and an outer helically wound spring element which is also flexible and when a correspondingly strong torque is applied, its diameter narrows so that it presses against the inner element from the outside and together they form a stiffened unit. In the case of this already known drilling equipment, this supple shaft is engaged immediately downstream of the hole mouth by the turning- and advance mechanism, so that only the section of the flexible shaft located within the drill hole is stiffened, while the section of the flexible shaft located behind the turning- and advance mechanism remains supple. This allows the drill bar to distort in any direction.
The latter drilling equipment did not yield satisfactory results in practice, because the rigidity of the stiffenable flexible shaft was not sufficient to transmit the advance forces and torque required for the drilling operation. Moreover, due to the friction of the insufficiently rigid flexible shaft against the wall of the drill hole the wear is extraordinarily high. Finally, the directional stability in this case is also poor, due to the strong advance forces to be transmitted and the thereto related deflections of the stiffened section of the flexible shaft.